| Abstrakt: |
A series of xanthobilirubinic acid (XBR) esters (i.e.,1c to1p) and amides (2a–2c) have been prepared by a procedure involving detachment of the chromophore previously ester-bound to a functionalized, insoluble polystyrene (1b). Detachment is achieved by treatment with the respective alcohol in aqueous alkali or with the amines, yielding directly the corresponding esters or amides. With primary, short-chained alcohols the ester yields are high (60% forn-C4H9-OH to 99% for C2H5-OH), but decrease rapidly with chain length (40% forn-C10H21-OH and 0% forn-C16H33-OH). The same trends are observed with the amines. These results are interpreted in terms of an (increasingly) unfavourable entropic interaction between the polymer matrix to which the chromophore is bound and the (growing) alcohol chain. The impairment by the long chained nucleophile to reach the transesterification center could also contribute, and, for the most lipophylic alcohols, their low solubility in aqueous alkali is an additional drawback. With secondary and tertiary alcohols, no ester is obtained, in agreement with a BAC2 mechanism involving a tetrahedral intermediate. |
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